Anti-flutter device for riser pipe



May 3, 1966 K. A. BLENKARN ANTI-FLUTTER DEVICE FOR RISER PIPE 2 Sheets-Sheet 1 Filed Aug. 23 1963 INVENTOR. KENNETH A. BLENKARN BY/DQQM ATTORNEY.

y 1966 K. A. BLENKARN 3,248,886

Filed Aug. 25, 1965 2 Sheets-Sheet 2 INVENTOR. KENNETH A. BLENKARN :1 M4 w FIG-2 ATTORNEY United States Patent 3,248,836 ANTI-FLUTTER DEVICE FDR RISER PIPE Kenneth A. Blenkarn, Tulsa, 01th., assignor to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed Aug. 23, 1963, Ser. No. 304,038 9 Claims. (CI. 61-46) This invention pertains to marine drilling operations carried out from a floating vessel or the like. In such marine operations, it is necessary to provide a tubular conduit and guide from the anchored drilling vessel to the top of the well at the sea floor. This is customarily taken care of by using what is called a riser pipe. This riser pipe is attached to the marine drilling equipment cemented in the ocean floor. Immediately above, there is a flexible joint which permits the upper end of the riser pipe to change in angular position relative to the bottom through an angle of some three to ten degrees. low the floating drilling vessel the riser pipe assembly includes a slip joint which permits the upper portion of this assembly to move longitudinally with respect to the lower position through a distance of feet or more. This telescoping joint is suitably packed off to insure no appreciable leakage of drilling fluid into the surrounding water or vice versa. Drilling operations are conducted inside this riser pipe.

It is apparent that it is desirable to maintain the riser pipe in a fixed position during the course of drilling. However, it has been found that in marine drilling operations conducted in the presence of strong ocean currents, for example when drilling a marine inlet, near a river mouth, or the like, the riser pipe develops a strong transverse vibration. Stated another way, the middle portion of the riser pipe tends to oscillate transversely with respect to the base and top, thus causing continual flexure of this pipe and of the drill pipe inside it. Such vibration can give rise to mechanical failure, considerably impeding the successful course of drilling.

It is an object of this invention to provide simple equipment which can be assembled on or around the riser pipe and thus prevent the occurrence of vibration due to currents in the water. This assembly permits the riser pipe type of drilling operation to be carried out in the presence of currents which would otherwise cause mechanical failure of this part ofthe system.

This invention is illustrated with the following drawmgs:

FIGURE 1 shows in diagrammatic view a cross section of the ocean at the location of a floating drilling vessel, showing the riser pipe'assernbly.

FIGURE 2 shows One form of an assembly (called a spoiler assembly) which illustrates an embodiment of my invention.

FIGURE 3 shows a cross sectional view of the equipment shown in FIGURE 2.

Many explanations have been advanced for the occurrence of vibration transverse to the longitudinal axis of the riser pipe during the course of marine drilling from a floating vessel. Experience early indicated that this vibration was associated with ocean currents impinging on the riser pipe. In FIGURE 1, there is shown a cross section through the ocean at the location of a floating drilling vessel 11 located at the surface 12 of the ocean. At some point which may be of the order of 100 feet or more below the drilling vessel is the ocean floor 13. Mounted on this ocean floor and normally cemented to it is an assemblage of drilling apparatus, which is shown in diagrammatic form as a block 14, since it merely illustrates the background in which my invention is situated. It may, and customarily will, include a suitable base and one or more blowout preventers, etc. Mounted on this is the Immediately be- 3,248,886 Fatented May 3, 1966 riser pipe assembly 15. At the lower part of this riser pipe assembly is the flexible joint 16 which permits angular deviation of the upper part of the riser pipe as mentioned above. At the upper part of the riser pipe assembly and customarily immediately below the base of the drilling vessel 11 is located the slip joint 17. The upper end of this is flexibly attached to the drilling vessel 11. A drill rig 18 is mounted on the drilling vessel 11, immediately above the top of the riser pipe assembly 15. The drill string is threaded through the riser pipe assembly 15. As drilling proceeds, drilling fluid is circulated through the drill string and back through the annulus between the string and the riser pipe assembly 15.. The drilling vessel is anchored by anchor lines (such as those shown at 19) from anchors (not shown).

It is apparent from an inspection of this figure that the riser pipe assembly below the slip joint 17 is a column in compression and that such column may be subject to considerable buckling stresses under the circumstances. The presence of transverse mechanical vibrations in this riser pipe assembly accordingly accentuates any tendency of the riser pipe assembly to bend or buckle. .At the same time such vibration, by application of an alternating stress, has a considerable tendency to cause the riser pipe assembly to fail under fatigue.

It has been found that the tendency of the riser pipe assembly to vibrate transversely increases as the current of water flowing by the assembly increases. Severe mechanical oscillation in a foot riser pipe has been observed in a current of about 7 mph. with, for example, an amplitude 25 feet down of approximately four inches at a frequency of about one-third to one-half cycle per second. Apparently, the riser pipe oscillates with a maximum amplitude more or less at the middle and substantially Zero amplitude at both ends.

One might assume that the principal problem would be that of dam-ping vibration and that, in the presence of the water, such vibration could be easily damped. However, this is not the main problem, which is that of preventing the formation of streams of eddies behind the riser pipe; that is, downstream from it. The presence of these eddies gives rise to an oscillating force transverse to the riser pipe. Accordingly, it is necessary to utilize a system minimizing the formation of these eddy currents.

I have found that it is possible substantially to prevent this transverse vibration due to currents by clamping to the riser pipe an apparatus which I call a flow control spoiler. This spoiler consists of a cylindrical perforated metal sheet or the like which is approximately 8 inches in diameter greater than the outer diameter of the riser pipe and which contains clamps holding the sheet to the riser pipe at intervals of the order of 4 to 6 feet. Thus, for example, in FIGURE 2 is shown one unit of the'flow control spoiler. This has a length L of the order of 10 feet and a diameter D approximately 8 inches greater than the diameter D of the riser pipe. The outer surface 20 of this spoiler is made of perforated sheet metal in which preferably the ratio of area of the holes to that of the remaining solid material is about one to one. This provides as much hole area as possible without too greatly weakening the structure. However, depending upon conditions, some benefit is gained if this ratio, instead of being approximately 1 to 1, is as much as 5 to 1 or as little as 1 to 5.

Any simple but strong clamping system may be employed to hold the perforated sheet about but radially separated from the pipe 15. In FIGURES 2 and 3, one form of such clamp is shown. The spoiler assembly is made in the form of two half-cylinders each containing a matching clamping half-ring 21, the inner diameter of which is that of the riser pipe 15 and the outer diameter of which is that of the perforated sheet 20. Three of 3 these rings 21 are shown in FIGURES 2 and 3, one at each end and one at about the middle. These are, of course, axially separated as they must be. They are firmly attached to the sheet, for example by welding, and are lined up so that three hinges 22 can be attached to the halves of the total assemblage. Completely opposite from the hinges 22 are three simple clamps 23 Welded to the half-rings 21. Accordingly, as the riser pipe assembly is made up, the spoiler unit is simply placed about the pipe and the halves clamped together, for example, by bolts 24. This firmly attaches one unit of the spoiler assembly about the riser pipe.

These assemblies may be made up with some space in between, but since it is desirable to minimize as far as possible the formation of the eddies, I prefer to butt each of these spoiler units against the adjacent ones so that there is a continuous outer cylindrical sheet of perforated metal extending substantially from the flex joint 16 to the lower end of the maximum extension of the slip joint 17, as shown in FIGURE 1. In any case, the perforate sheets of such spoiler units should cover a major part of the pipe exposed to the current.

The thickness of the perforated plate is not particularly critical. In one such assembly which has been tested, the plate was approximately /4 inch thick With /2 inch holes punched through it. It is desirable to have the edges of the perforations as sharp as possible.

It is apparent that numerous modifications and changes may be made in the design of each of the spoiler units making up the assembly without varying from the major design which has been shown. Thus, for example, it is possible to substitute material such as chain link fencing for the perforated plate. Transverse metal grating could be employed satisfactorily. However, it is still desirable to maintain a ratio of hole area to remaining sheet area of approximately 1 to 1 if possible. Again, the spacing between the riser pipe and the spoiler sheet is not particularly critical. While a spacing of approximately 4 inches has been found desirable, this spacing may be as little as 2 inches or as great as 6 inches or more. The particular clamping assemblage shown is merely one of many that can be made up, the principal design consideration being to hold the perforated spoiler sheet substantially fixed in place relative to the outer surface of the riser pipe.

I claim:

1. An assemblage for minimizing current induced transverse vibration in a cylinder extending upward from and attached to the floor of a body of water which comprises at least one cylindrical perforated sheet surrounding said cylinder, the total area of perforated sheet covering at least a major part of said cylinder, and

a plurality of axially substantially separated supports radially attaching said perforated sheet firmly to but' separated from said cylinder, said supports forming the ratio of area of perforations to the area of remaining sheet lying between approximately 1:5 I

and 5:1, and at least two substantially axially separated radial supports, one near each end of each spoiler mounting said perforatedsheet firmly upon but I radially separated from said pipe,

said plurality of said spoilers covering at least a major part of said pipe, but without substantial obstruction to water waves in both axial and circumferential directions.

3.- Apparatus in accordance with claim 2 in which the radial spacing from said sheet to the outer surface of said pipe is about 2 to 6 inches.

4. Apparatus in accordance with claim 2 in which the radial spacing from said sheet to the outer surface of-said pipe is about 2 to 6 inches and said ratio is approximately 1:1.

5. An assemblage for minimizing current-induced vibration in a cylinder surrounded by a liquid which comprises a cylindrical perforated sheet surrounding said cylinder,

and

a plurality of axially separated supporting rings the inher diameter of which is substantially the outer diameter of said cylinder, said rings being firmly attached to-said sheet with at least one of said rings near each end of said sheet, said rings being axially separated by a distance substantially greater than the difference in outer diameter of said sheet and said cylinder and furnishing the only obstruction to axial flow of liquid between said sheet and said cylinder,

said sheet and said rings being axially split along a plane passing through the cylindrical axis of said sheet,

a plurality of hinges connecting one set of adjacent ends of said rings, and

a plurality of clamps mounted on the opposite set of ends of said rings from said hinges, whereby said assemblage may be mounted around and clamped to said cylinder.

6. Apparatus in accordance with claim 5 in which the radial spacing of each of said rings is about 2 to 6 inches.

7. Apparatus in accordance with claim 5 in which the radial spacing of each of said rings is about 4 inches.

8. Apparatus in accordance with claim 5 in which the radial spacing of each of said rings is about 2 to 6 inches and the ratio of area of perforations of said sheet to the remaining area of said sheet lies between about 1:5 and 5:1.

9. Apparatus in accordance with claim 5 in which the radial spacing of each of said rings is about 2 to 6 inches and the ratio of area of perforations of said sheet to the remaining area of said sheet is approximately 1:1.

References Cited by the Examiner UNITED STATES PATENTS 1,157,328 '10/1915 Scowden 61-54 2,475,888 7/1949 Hackett 6146 CHARLES E. OCONNELL, Primary Examiner. I. SHAPIRO, Assistant Examiner.- 

1. AN ASSEMBLAGE FOR MINIMIZING CURRENT-INDUCED TRANSVERSE VIBRATION IN A CYLINDER EXTENDING UPWARD FROM AND ATTACHED TO THE FLOOR OF A BODY OF WATER WHIHC COMPRISES AT LEAST ONE CYLINDRICAL PERFORATED SHEET SURROUNDING SAID CYLINDER, THE TOTAL AREA OF PERFORATED SHEET COVERING AT LEAST A MAJOR PART OF SAID CYLINDER, AND A PLURALITY OF AXIALLY SUBSTANTIALLY SEPARATED SUPPORTS RADIALLY ATTACHING SAID PERFORATED SHEET FIRMLY TO BUT SEPARATED FORM SAID CYLINDER, SAID SHEET AND SAID CYLTHE SLOE CONNECTION BETWEEN SAID SHEET AND SAID CYLINDER. 